We consider the magneto-electric optical properties of the quantum vacuum and show that all the different phenomenona are related by Lorentz invariance. As a model calculation we show how crossed fields properties can be calculated starting from single field properties by using Lorentz transformations. Using this method we have studied for the first time the case of a crossed static magnetic field and electric field applied with one of these two fields parallel to the direction of light propagation. We also show that parallel field properties can be found using general symmetry properties.